1. Field of the Invention
The present invention relates to a thin-film semiconductor having crystallinity and also to a method of fabricating such a thin-film semiconductor. Furthermore, the invention relates to a semiconductor device utilizing such a thin-film semiconductor. In addition, the invention relates to a method of fabricating this semiconductor device.
2. Description of the Related Art
Techniques for forming a crystalline silicon film on a glass substrate or quartz substrate and fabricating thin-film transistors (TFTs) from this silicon film are known.
These TFTs are known as high-temperature polysilicon TFTs or low-temperature polysilicon TFTs.
High-temperature polysilicon TFT fabrication is a technique utilizing a heat treatment conducted at relatively high-temperatures such as 800° C., 900° C., and more, in fabricating crystalline silicon films. It can be said that this technique has derived from IC fabrication processes making use of single-crystal silicon wafers.
Of course, quartz substrates withstanding the above-described heating temperatures are used as substrates on which high-temperature polysilicon TFTs are formed.
On the other hand, low-temperature polysilicon TFTs are fabricated on cheaper glass substrates which are, of course, inferior in heat resistance to quartz substrates.
When a crystalline silicon film forming low-temperature polysilicon TFTs is fabricated, a heat treatment at a temperature lower than 600° C. is used so that the glass substrate can stand up to it, or laser annealing technique which hardly thermally damages the glass substrate is exploited.
High-temperature polysilicon TFT fabrication technology is characterized in that it can integrate TFTs having uniform characteristics on a substrate at a high density.
On the other hand, low-temperature polysilicon TFTs can make use of glass substrates which are cheap and can easily provide large areas.
It is to be noted that with the current technology, high-temperature polysilicon TFTs do not differ greatly from low-temperature polysilicon TFTs in characteristics. The slight differences between them are that high-temperature polysilicon TFTs are superior in production yield and uniformity of characteristics across the substrate while low-temperature polysilicon TFTs are superior in productivity and production cost.
With respect to the characteristics, both kinds of TFTs show mobilities of approximately 50 to 100 cm2/V s and S values of about 200 to 400 mv/dec (VD=1 V).
These characteristics mean that these TFTs can operate at speeds about two orders of magnitude higher than those of TFTs using amorphous silicon. However, the characteristics of the high-temperature polysilicon TFTs are much inferior to those of MOS transistors using single-crystal silicon wafers. Generally, the S values of MOS transistors employing single-crystal silicon wafers are on the order of 60 to 70 mV/dec. Furthermore, their operating frequencies are 1-2 orders of magnitude higher than those of high- and low-temperature polysilicon TFTs.
At present, TFTs are used to integrate the active matrix circuit of an active matrix liquid crystal display and its peripheral driver circuit on the same substrate. That is, the active matrix circuit and peripheral driver circuit are fabricated from TFTs on the same substrate.
In this configuration, the source driver circuit of the peripheral driver circuit is required to operate considerably above 10 MHz. Today, however, with respect to a circuit composed of high- and low-temperature polysilicon TFTs, a margin of only several megahertz is given to the operating speed.
Accordingly, it is customary to multiplex the operation, so that a matrix-driven liquid crystal display is fabricated. However, this method has the disadvantage that stripes appear on the viewing screen due to subtle deviations of timing of multiplexing.
A conceivable forthcoming technique is to integrate oscillator circuits, D/A converters, A/D converters, and digital circuits for performing various kinds of image processing on the same substrate, in addition to the peripheral driver circuit including a shift register circuit and a buffer circuit.
However, it is necessary that the aforementioned oscillator circuits, D/A converters, A/D converters, and digital circuits for performing various kinds of image processing operate at higher frequencies than the peripheral driver circuit.
Therefore, it is substantially impossible to fabricate these circuits from the high- and low-temperature polysilicon TFTs obtained by the current techniques.
It is to be noted that an integrated circuit which consists of MOS transistors using a single-crystal silicon wafer and can be operated above 100 MHz has been put into practical use.